This is usually caused by a bad ground, especially on an older bike. Try running a wire direct from the brake light metal case to the negative battery terminal and see if the light is bright then. Checking with a volt meter is good but you must check the voltage with the meter grounded at the light. This way you might see low voltage and then move the meter ground lead to the battery and you will see it increase. If a new ground wire does the trick you can find a way to run it out of sight near the original harness.